Anesthetic concentrations of pentobarbital markedly reduced the fast nicotinic EPSP while having no effect on the slow EPSP or slow IPSP of frog sympathetic ganglia even though all three synaptic potentials depend on the presynaptic release of acetylcholine. A similar differential effect was seen for nicotinic and muscarinic responses to exogenously applied agonists, while the depolarizing action of gamma aminobutyric acid was enhanced. These results indicate that pentobarbital has remarkably selective action on the sympathetic ganglion and further indicate that blockade of ganglionic transmission by anesthetic concentrations of pentobarbital can be entirely explained by a postsynaptic action.